Reflectance measuring apparatus measure the proportion of the radiation reflected from a specimen to the radiation reflected from a comparison specimen under the same conditions. For this purpose, the specimen is placed on a circularly-shaped measurement opening in many such apparatus so that the specimen is received by the measuring apparatus in a precisely defined position. Often, this measuring aperture is arranged in a so-called Ulbricht sphere by means of which the specimen is diffusibly illuminated.
With some reflectance apparatus, a so-called measuring head having all parts necessary for the measurement in the direct vicinity of the specimen is separated from the remainder of the apparatus and is connected to the latter only via a cable. In these apparatus, the measuring aperture can be brought to the specimen which is an advantage especially for large and heavy specimens. Such a reflectance measurement apparatus is described, for example, in the article entitled "Diode Array Spectrometer: an Optimised Design" by H. H. Schlemmer and M. Machler, J. Phys. E: Sci-Instrum., Volume 18, pages 913 to 919, 1985.
The measurement opening must not exceed a specific diameter so that the measuring apparatus or the measuring head is not too large. However, since the measuring aperture must be large with respect to the surface structure of the specimen in order to obtain measurement results which are independent of which part of the specimen is by chance applied to the measuring aperture, only specimens with a sufficiently small surface structure can be reliably measured for a predetermined diameter of the measuring aperture. This is especially then a disadvantage when specimens having a large surface structure are to be measured only from time to time.
Further, an enlargement of the measurement surface on the specimen cannot be obtained by arranging the specimen in a correspondingly large spacing from the measuring aperture because not only does the illumination of the specimen become so weak that the reflected radiation is no longer adequate for measuring but also because the angular distribution of the rays incident on the measurement surface of the specimen is so changed even with small distance changes that the measurement values and therefore also the color range change considerably.